Forum for Science, Industry and Business

At Molecular Scale, Vibrational Couplings Define Heat Conduction

23.09.2004

Too much heat can destroy a sturdy automobile engine or a miniature microchip. As scientists and engineers strive to make ever-smaller nanoscale devices, from molecular motors and switches to single-molecule transistors, the control of heat is becoming a burning issue.

The shapes of molecules really matter, say scientists from the University of Illinois at Urbana-Champaign and the University of Scranton who timed the flow of vibrational heat energy through a water-surfactant-organic solvent system. The rate at which heat energy moves through a molecule depends specifically on the molecule’s structure, they found. "The flow of vibrational energy across a molecule is dependent upon where and how the energy is deposited," said Dana Dlott, a professor of chemistry at Illinois and a co-author of a paper to appear in the journal Science, as part of the Science Express Web site, on Sept. 23. "Unlike normal heat conduction, different excitations may travel across the molecule along different paths and at different rates."

The system the scientists studied is called a reverse micelle, and consisted of a nanodroplet containing 35 water molecules enclosed in a sphere of surfactant (sodium dioctyl sulfosuccinate) one molecule thick that was suspended in carbon tetrachloride. The ultrafast laser technique, developed at Illinois, monitored vibrational energy flow as it moved from water, through the surfactant shell out to the organic solvent, atom by atom.

When the researchers deposited energy in the nanodroplet, the vibrations moved through the surfactant and into the carbon tetrachloride within 10 picoseconds. However, when the energy was deposited directly into the surfactant, the vibrations required 20 to 40 picoseconds to move into the carbon tetrachloride. Even though the distance was shorter, the energy transfer took significantly longer. "This is opposite of what you would think in terms of simple and ordinary heat conduction," Dlott said. "To explain this strange result, we have to analyze the energy transfer in terms of specific vibrational couplings that occur through a vibrational cascade."

There are hundreds of different vibrations in the water-surfactant-organic solvent system, Dlott said. "When energy moves through molecules, the detailed structure of the molecules and the way the vibrations interact are extremely important."

When the water was excited by a laser pulse, the scientists report, much of the energy was immediately moved to the surfactant, which then efficiently transferred the energy to the carbon tetrachloride. But when the surfactant was excited by the laser, the energy took a different path among the atoms, delaying the transfer to the carbon tetrachloride.

"The movement of vibrational energy within and between molecules is a fundamental process that plays a significant role in condensed matter physics and chemistry," Dlott said. "In designing nanoscale devices, the shapes of the molecules must be designed not only to be small and fast, but also to efficiently move heat."

The National Science Foundation, the Air Force Office of Scientific Research and the U.S. Department of Energy supported this work.

Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.

Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....

Proteins must be folded correctly to fulfill their molecular functions in cells. Molecular assistants called chaperones help proteins exploit their inbuilt folding potential and reach the correct three-dimensional structure. Researchers at the Max Planck Institute of Biochemistry (MPIB) have demonstrated that actin, the most abundant protein in higher developed cells, does not have the inbuilt potential to fold and instead requires special assistance to fold into its active state. The chaperone TRiC uses a previously undescribed mechanism to perform actin folding. The study was recently published in the journal Cell.

Actin is the most abundant protein in highly developed cells and has diverse functions in processes like cell stabilization, cell division and muscle...

Scientists have discovered that the electrical resistance of a copper-oxide compound depends on the magnetic field in a very unusual way -- a finding that could help direct the search for materials that can perfectly conduct electricity at room temperatur

What happens when really powerful magnets--capable of producing magnetic fields nearly two million times stronger than Earth's--are applied to materials that...

The quality of materials often depends on the manufacturing process. In casting and welding, for example, the rate at which melts solidify and the resulting microstructure of the alloy is important. With metallic foams as well, it depends on exactly how the foaming process takes place. To understand these processes fully requires fast sensing capability. The fastest 3D tomographic images to date have now been achieved at the BESSY II X-ray source operated by the Helmholtz-Zentrum Berlin.

Dr. Francisco Garcia-Moreno and his team have designed a turntable that rotates ultra-stably about its axis at a constant rotational speed. This really depends...